XPS15 9570 throttles CPU to 0,78Ghz on high network traffic while CPU/GPU idle!

I've had a long history with this machine throttling to being unusable. Recently, we're doing backup snapshots of our machines and pushing these through the network to our backup media.

That should be a process running in the background, not using much system ressources. In my case, I can only hardly use my Laptop anymore while the snapshot is being uploaded. Even the mouse is lagging while the CPU takes a break at 0,78Ghz. As you can see in the screenshots, Package Power is at 18W all the time, thermals are perfectly ok at around 50 to 60 degrees (CPU / GPU). No component is drawing much power.

- CPU Turbo is disabled in BIOS, so the highest possible CPU Clock is 2,9Ghz
- Energy saving plan: COnfig TDP-Level is set to "nominal TDP" for both
- Dell Update does not propose component updates. BIOS is the most recent version

I attached some screenshots showing what is going on:

Something is seriously wrong with this machine. Does anyone have an idea?